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The Open Fuels & Energy Science Journal, 2015, 8, 1-13 1 Open Access

Numerical Investigation on the Flow, Combustion and NOx Emission Characteristics in a 10 MW Premixed Gas Burner Weibo Chen and Guixiong Liu*

School of Mechanical & Automotive Engineering, South China University of Technology, Guangzhou, China

Abstract: The characteristics of the combustion temperature, flow velocity, CO distribution and NOx emissions of a 10 MW gas burner at different primary to secondary air ratios are numerically studied using computational fluid dynamics software Fluent. The results indicate that the primary to secondary air ratio in gas burner determines the combustion quality through influencing some parameters directly, such as the combustion efficiency, combustion intensity, profile and stability of flame as well as emission of NOx. Then two evaluation indexes of combustion quality are summarized after analyzing the flame structure and characteristics of the flow. The detailed results reported in this paper may provide a useful basis for NOx reduction and premixed gas burner design. Finally some proposals are given to choose the optimal primary to secondary air ratio for a gas burner. Keywords: Combustion, computational fluid dynamics, energy, numerical simulation..

1. INTRODUCTION In this paper, a kind of premixed gas burner produced was taken for instance. Combustion situations inside a boiler Gas burners are a kind of device which jet the flaming furnace at different primary to secondary air ratios were gas mixed with fuel gas and air. They are used necessarily in simulated using CFD software Fluent. The characteristics of heating energy and related industries, for example, boilers, temperature, flow velocity, NOx distribution and flame smelting furnaces and thermal treatment. It controls thermal profiles have been analyzed to study how the primary to load, temperature distribution, thermal efficiency and secondary air ratio affects the combustion quality. The working life of the controlled object because it’s the core of results have important significance for both combustion the whole heating equipment [1-8]. Some experimental quality and burner structure improvement. results indicate that expansive shape of fuel is significantly influenced by the structures and dimensional parameters of fuel injection systems [2]. Fuel jet velocity and air velocity 2. NUMERICAL ANALYSIS PROCEDURE affect the blow-off behavior of Swirl-Stabilized Premixed, 2.1. Flow Fields of Burner Non-Premixed and Spray Flames [5]. When the burners are working, combustion efficiency and intensity of fuel and The structure of premixed gas burner is shown stability of flame have decisive influences on combustion schematically in Fig. (1a). Its air-inlet is a rectangle which quality. And these parameters above depend on the supply has a width of 0.400 m and a length of 0.711 m. The nozzle situation of oxygen in the combustion process [9-12]. The of burner is a circle with a diameter of 0.340 m. In order to unsatisfactory situation of oxygen supply may be due to lack reduce computational domain for computation, air flow field of air or inappropriate proportion of primary air which mixed inside the whole burner has been simulated by CFD software with fuel gas and secondary air participating in combustion. before the combustion simulation. Then appropriate primary So in a certain amount of total air, the primary to secondary to secondary air ratios as well as air velocity would be air ratio has a remarkable influence on the flame chosen as boundary conditions for combustion numerical temperature, flow velocity distribution and NOx emission. simulation according to the simulation results of the air flow Technology of computational fluid dynamics (CFD) is field. Calculation models of three opening degrees (15°, 30°, wildly used in the research field of thermal energy such as 45°) of the secondary air door are built for numerical combustion studying [9, 13-15]. Different kinds of models simulation according to the structure of the gas burner. The and methods are utilized to simulate all kinds of value of coefficient of excess air is 1.08. The volume flow characteristics of flames [1, 7, 15]. It is necessary to simulate rate of air actually needed is 2.8866 m3/s when the gas the combustion characteristics of the gas burner since CFD burner is working nearby the rated thermal load. technology can provide more details of the flow field than Fig. (1b-d) shows the situations of the air velocity experiment. distribution with different opening degrees of the secondary air door. The air speed inside the air door decreases as the

opening degree of the secondary air door increases. And *Address correspondence to this author at the School of Mechanical & there are some distinctions of velocity distribution. When the Automotive Engineering, South China University of Technology, opening degree is 15°, air velocity nearby the secondary air Guangzhou, China; Tel: +86-20-87110568; Fax: +86-20-87110568; E-mail: [email protected] door is the highest and it decreases down along the outlet

1876-973X/15 2015 Bentham Open 2 The Open Fuels & Energy Science Journal, 2015, Volume 8 Chen and Liu

Fig. (1). Structure of the premixed gas burner (a) and velocity distributions of airflow with different opening degrees of the secondary air door (b) (c) (d). down to a minimum velocity of about 9.18 m/s near the 38.50 m/s. A swirling flow formed via the air flow passing primary air pipe in the lower area of secondary air door. The the swirler is found in the middle of the area between the highest speed (78.10 m/s) appears nearby the secondary air swirler and the nozzle. It continuously absorbs flue gas from door. When the opening degree is 30°, the total velocity the outlet. The velocity distribution of air when it enters into distribution is not the same as the situation of 15°. Air speed combustion chamber is relatively uniform. Result details near air door is relatively low. It increases first and falls a bit show that only primary to secondary air ratios and the toward the outlet. The maximum value of velocity appearing swirler has remarkable influences on the situations of the in the upper area of the combustion chamber is about 54.20 airflow ejected from the burner. This phenomenon should be m/s. It increases obviously after passing through the swirler. considered to simplify the computational domain in When the opening degree is 45°, the total velocity combustion simulation. The primary to secondary air ratios distribution is similar to the situation of 30°, but the average under conditions of different opening degrees of secondary velocity is lower. The highest speed appears in the area near air door can be obtained by calculating the mass flow of air the secondary air door. After passing through the swirler, the passing into the primary pipe and secondary air inlet air velocity increases slowly and achieves to top speed of mechanism respectively.

Numerical Investigation on the Flow, Combustion and NOx Emission The Open Fuels & Energy Science Journal, 2015, Volume 8 3

2.2. Model Building and Meshing After the grid-independency of the results was verified using a finer mesh with 18,118,314 cells, which had been Since gas burner is a key component of the combustion indicated by the fact that the results (including the flow system, its most direct effect is to heat the boiler furnace. structure, velocity distribution, temperature iso-contours of When the burner is working, flame is spraying out from the 2000 K) for the case of primary to secondary air ratio combustion chamber into the boiler furnace, so most of the 7.925% using the two different meshes were kept in high flames, high temperature flue gas and NOx pollutants are consistency (shown in Fig. 3), the primary mesh was finally generated in the furnace. Establishing burner-combustion employed for solving all other cases. chamber model to simulate the normal operation of the burner can make the computed result more close to the real situation. The thermal load of premixed gas burner used 2.3. Computational Models throughout this paper is 10 MW. The shape and size of the The combustion simulations were carried out using the furnace model were determined based on the 12 t/h boiler commercial CFD package ANASYS FLUENT. Numerical equipment. In order to facilitate the study of the impact of calculations of the present problem included solutions of the the primary air, secondary air and the fuel flow to the Favre-averaged form of mass, momentum, energy, species, combustion quality, considering the velocity distribution of turbulent kinetic energy k, and its dissipation rate ε transport air inside the burner obtained by simulation in the previous equations available in FLUENT. section simultaneously, the simulation model shown in Fig. (2a) only contains the combustion chamber of burner and In addition, the realizable k−ε model with the standard boiler furnace. Fig. (2b) exhibits the magnification of wall function was employed to predict the turbulence in the combustion chamber of the simplified model. And combustion system. In this paper, the non-premixed model representative primary to secondary air ratios were was used for combustion, a mixture-fraction equation is determined by the characteristics of the air flow field solved instead of equations for individual species, and the simulated. SolidWorks 2012 and ICEM software were used individual species concentration is derived from the to build the model and create triangular unstructured grids predicted mixture fraction concentration under the respectively. Moreover, the body-fitted meshing technique assumption of chemical equilibrium. Interaction between was employed. In the combustion chamber of the burner and turbulence and chemistry is accounted for with the β- region near the burner jet, finer grids were generated, while function probability density function (PDF). The SIMPLE in regions far away from the flame base and the axis coarser algorithm was used to solve the pressure-velocity coupling, ones were generated. In this way, a primary mesh with a and a second-order discretization scheme was utilized to total of 9,118,314 tetrahedral cells was generated (shown in solve all governing equations. Fig. 2c).

Fig. (2). Schematic of the burner-combustion chamber model (a) (b) and mesh system of the model (c). 4 The Open Fuels & Energy Science Journal, 2015, Volume 8 Chen and Liu

Fig. (3). Verification of grid-independency for case of primary to secondary air ratio 7.925%. (The upper is for the coarse grids with 9,118,314 cells and the lower is for the fine grids with 18,118,314 cells. The velocity distribution is shown in rainbow color. The streamlines, temperature isocontours of 2000 K (the black solid line) are superimposed on it).

2.4. CH4-Oxidation and NOx-Formation Modeling ∂  ρY +∇ ⋅ ρvY =∇ ⋅ ρD∇Y +S (7) t ()NO ()NO ()NO NO CH4 combustion reactions are highly complex, and are ∂ normally simplified into a series of global reactions with Table 1. Analysis data of the liquefied natural gas used for CFD modeling. A typical reactions scheme can be simulation. summarized in the form [16]: 1 Parameter Value CH + O → CO+2H (1) 4 2 2 2 CO2 (w%) 0.5 1 CO+ O → CO (2) CO (w%) 0.1 2 2 2 CH4 (w%) 95 1 H (w%) 1 H + O → H O (3) 2 2 2 2 2 N2 (w%) 1 species After the combustion calculation, the NOx calculation is O2 (w%) 0 performed based on the predicted temperature and species CmHn (w%) 2.4 concentrations under the assumption that the NOx 3 concentration is very low and, therefore, has a negligible H2S (mg/m ) 400 impact on the natural gas combustion. NOx is formed mainly 3 H2S (mg/m ) 100 by thermal NOx formation mechanisms. A transport equation 3 for the NO species (i.e., NO, NO2, N2O) is calculated, which H2O (mg/m ) 0.2-2 takes into consideration convection, diffusion, production lower dw Q 35588 H S (kJ/m3)/8850(H S (kcal/m3)) and consumption of NO species. calorific value ar 2 2

k ,1 standard f (kg/m3) 0.718 density ρ O+N 2  N+NO (4) k ,1 r k ,2 f where N+O2 O+NO (5) k ,2 Y = mass fraction of NO species in the gas phase r NO D=effective diffusion coefficient kf ,3 N+OH H+NO (6)  SNO=source term for NO species. k ,3 r The net rate of formation of NO species via Eqs. (4)-(6) where is given by kf = forward reaction rates. d ⎡NO⎤ k = reverse reaction rates. ⎣ ⎦ = k ⎡O⎤⎡N ⎤+k ⎡N⎤⎡O ⎤+k ⎡N⎤⎡OH⎤ r dt f ,1 ⎣ ⎦⎣ 2 ⎦ f ,2 ⎣ ⎦⎣ 2 ⎦ f ,3 ⎣ ⎦⎣ ⎦ (8) The value of kf and kr are determined based on the evaluation -k ⎡NO⎤⎡N⎤-k ⎡NO⎤⎡O⎤-k ⎡NO⎤⎡H⎤ of Hanson and Salimian [9]. r,1 ⎣ ⎦⎣ ⎦ r,2 ⎣ ⎦⎣ ⎦ r,3 ⎣ ⎦⎣ ⎦ 3 For the above thermal NOx mechanisms, only a NO where all concentrations have units of gmol/m . The species transport equation is required: concentrations of O, H, and OH are calculated by the partial equilibrium approach [17]. Numerical Investigation on the Flow, Combustion and NOx Emission The Open Fuels & Energy Science Journal, 2015, Volume 8 5

2.5. Calculation Conditions Setting hydraulic diameters of one single annular gas pipe outlet and central gas tube outlet are 6 mm and 5 mm respectively. According to the situations of flow field under the There are total 80 annular gas pipe outlets at the end of the conditions of three different secondary air door opening annular gas pipe and 12 central gas tube outlets at the end of degrees obtained by the previous numerical simulation, in the central gas tube in the burner. order to better analyze the impact of different primary to secondary air ratios to combustion quality, the proportions The calculation formulas of velocity, turbulence intensity selected to research are 15.000%, 7.925% and 5.985%, and and hydraulic diameter are determined as Eqs. (11), (12), and the coefficient of excess air is 1.08. The fuel used in the (13) respectively: simulation is liquefied natural gas (LNG) and the parameters V of LNG used are shown in Table 1. v = 0 (12) 0 A The first step in setting the boundary conditions is calculating the actual amount of air to reach the thermal load S D 4 (13) of 10 MW of burner according to the formulas (9)-(10). The H = × P fuel consumption rate of 0.2838 m3/s has been calculated −1 8 when the burner was designed, and the coefficient of excess ⎛ v D ⎞ air α is defined as 1.08. I = 0.16 × 0 H (14) ⎝⎜ ν ⎠⎟ Theoretical amount of air required for the gas combustion is calculated as follows: where 3 dw V0 = volume flow rate of gas needed (m /s) 0 Qar V = 0.264 + 0.02 (9) 3 1000 A = sectional area of outlet (m ) S = infiltration cross-sectional area of pipe (m3) where Qdw represents the lower calorific value of natural gas. ar Actual amount of air required for the gas combustion is P = perimeter of infiltration cross-sectional area of pipe (m) calculated as follows: ν = kinematic viscosity of gas (m2/s) V = αV 0 (10) After setting boundary conditions in accordance with three different proportions of primary air in the total amount where α is the coefficient of excess air. The actual amount of of air 5.985%, 7.925% and 15.000% respectively, the air required unit time is determined as follows: FLUENT solver was started to simulate. V 0 = V ⋅ V (11) k r 3. RESULTS AND DISCUSSION where V is the volume of fuel consumed. r 3.1. Validation of the Numerical Results Setting the boundary conditions need to calculate velocities, turbulence intensities and hydraulic diameters of To validate the simulation results, the predicted velocity the primary air pipe and secondary air mechanism outlet, and species profiles were compared with the measured data. central gas tube outlet and annular gas tube exit. The three Fig. (4) shows the radial profiles for the velocity and O2 volumes are calculated according to Eqs. (12)-(14). And all mass fractions at line A-B which is 10 cm apart from the desired boundary conditions are obtained through burner nozzle at the center cross-section of the furnace considering formulas (9)-(10) simultaneously. (shown in Fig. 5b). The results are shown in Table 2. The parameters in the Results of the error analysis indicated that the average table are used as boundary conditions of the simulation. The relative errors for the predicted velocities and O2 mass

Table 2. Boundary conditions for combustion simulation.

Boundary Proportion of Outlet of Outlet of Outlet of Annular Outlet of Central Conditions Primary Air (%) Primary Air Secondary Air Gas Pipe Gas Tube

5.985 29.08 20.05 Airflow velocity (m/s) 7.925 38.5 19.63 115.47 66.54 15.000 72.87 18.12 5.985 4.38 3.44 Turbulence intensity 7.925 4.19 3.45 4.14 4.54 (%) 15.000 3.85 3.49 5.985 Hydraulic diameter 7.925 32.00 196.80 6.00 5.00 (mm) 15.000 6 The Open Fuels & Energy Science Journal, 2015, Volume 8 Chen and Liu

Fig. (4). Comparison of the measured and calculated velocity (a), O2 mass fraction (b) distributions along the line A-B (Lines represent the predicted results and scattered symbols represent the measured data). fractions were kept within ± 4.3% and ±1.9% respectively. A conclusion is proposed that the effect of the The predictions agreed well with the measurements. This combustion is the best when the proportion is 15.000% and means that the adopted numerical models in the present the worst when the proportion is 5.985%. calculations are reasonable for combustion analysis and NO x Variations of velocity of line A-B is shown in Fig. (4a). emissions in the boiler. When the proportion of primary air is 5.985%, the velocity in the central area is the slowest, however, the strength of the 3.2. Velocity Distributions swirling flow is insufficient and the capacity to suck the high temperature flue gas is poor. So the combustion here is the Velocity distributions inside the boiler furnace under most complete and it creates a maximum of flue gas. This conditions of different primary to secondary air ratios are makes the fastest speed in the central region among three shown in Fig. (5). There are some similarities of the flow proportions. Since the amount of secondary air outside the field inside the furnace. A swirling flow is found between flames is the highest, the total speed in line A-B is the fastest the inner and outer layers of flames. This area has the highest when the proportion of primary air is 5.985%. velocity of above 172 m/s and the temperature of 2000 K above. The O2 concentration here is almost zero. This is explained by the fact that the gas here is high-temperature 3.3. Temperature Distributions flue gas sucked back in front of the flames. It can ignite the Fig. (6) shows the temperature distributions in the fresh mixture air to stabilize flames. horizontal cross section of the boiler furnace. The flames Meanwhile, there is also a swirling flow inside the inner profile can be demonstrated by analyzing simulation results. flames, but its speed of about 120 m/s and temperature of The flames are divided into inner and outer flames both have about 1500 ~ 1700 K are relatively low. doughnut-shapes. The outer flames are formed by the fixed gas sprayed from swirler. This part of gas consists of the Rules of how primary to secondary air ratio affects flame liquefied natural gas jetted from the nozzle of the annular gas shapes are summarized through comparing Fig. (5c) which show velocity profiles under conditions of different primary pipeline and the secondary air. Since there is a small amount of fuel but excessive oxygen in the outer flames, the to secondary air ratios. In the range under blow-off limit, as combustion reaction was complete so quickly. The outer the quantity of primary air increases, both the maximum flames have a shorter length which only about half of the speed and overall speed of flames increases. The velocity inner flames. And their temperature is lower. The inner and swirl strength of the inner flames is also increasing. The flames are formed by fixed gas which consists of small peripheral contour of flames is shrinking and the heating zone becomes more concentrated while the capacity of amounts of gas ejected from the annular gas pipeline and sucked by the swirling flow in front of the combustion swirling flow to suck high temperature flue gas is enhancing. chamber outlet and gas ejected from the central gas tube and So flame stability is improved. Characteristics of velocity are primary air. This part of the flames has a longer combustion similar to the flame temperature distributions analyzed in process and flame length and higher temperature due to the section 3.3. When the proportion of primary air is 15.000%, sufficient fuel. It could be regarded as the main flame flame combustion efficiency, stability and intensity of flames are optimum among the three proportions. heating the furnace. Fig. (6) also shows the temperature is lower in the central portion of the flames. Because the center of the flame contains excess fuel, the oxygen content is Numerical Investigation on the Flow, Combustion and NOx Emission The Open Fuels & Energy Science Journal, 2015, Volume 8 7

Fig. (5). Velocity distributions at different primary to secondary air ratios. relatively lower. There is almost no combustion reaction Variations of temperature of line A-B which is 10 cm occurring in this area. The more outward from the centre, the apart from the combustion chamber outlet at the center cross- O2 concentration relatively increase and the combustion section of the furnace is shown in Fig. (7a). When the reaction becomes more drastic. The temperature rises up proportion of primary is 15.000%, oxygen content in the until the O2 concentration is zero. The temperature reaches center of the flame is higher than that of proportions of the highest value when the reaction is stopped. Outside the 7.925% and 5.985%, so it has the fastest combustion flames, temperature of furnace maintained the maximum efficiency and the highest temperature among three of them. value of about 2240 K until a sharp decrease appears in the When the proportion of primary is 5.985%, the oxygen exit of the furnace. content is less than that of 7.925%, but the slower velocity In addition to the common characters of gas burning makes more time for mixing oxygen and gas in this region, so combustion reaction is faster. Temperature outside the flame analyzed above, rules of how primary to secondary air flames is much higher than that of the flame center. From the ratio affects flame temperature distributions are summarized figure, sizes of the flame structure are obtained. Shape of through comparing Fig. (5a-c) which show velocity profiles inner flames presents a circle with an inner radius of 0.02 m under conditions of different primary to secondary air ratios. and an external radius of 0.14 m. Center of this circle is at As the proportion of primary air increases, the amount of air mixed with the gas out of center gas pipe becomes larger and the middle of the combustion chamber outlet. The shape of the outer flames which has an inner radius of about 0.14 m combustion reaction of the inner flames is accelerated. The and an outer radius of 0.28 m is similar to that of the inner length of flames becomes shorter and the region which has a flames. Maximum value of flame temperature is 2230 K. lower temperature inside the furnace gradually becomes lathy, indicating that the flames focus and better heat the Through analyzing the temperature distributions in this furnace. line and the results of flame shapes analyzed in section 3.2, 8 The Open Fuels & Energy Science Journal, 2015, Volume 8 Chen and Liu

Fig. (6). Temperature distributions at different primary to secondary air ratios. primary to secondary air ratios affect the combustion 3.4. Distributions of the CO Concentration reaction by changing the oxygen content and velocity distribution of gas. Thereby the shape of the flames can be As intermediate products CO is produced during the controlled. combustion process, the rule of how combustion reaction is

Fig. (7). Variations of temperature and CO mass fractions along line A-B shown in Fig. (5b) at different primary to secondary air ratios. Numerical Investigation on the Flow, Combustion and NOx Emission The Open Fuels & Energy Science Journal, 2015, Volume 8 9

Fig. (8). Mass fraction distributions of CO at different primary to secondary air ratios. affected by different primary to secondary air ratios is There comes to a conclusion that the oxygen analyzed through observing the distributions of the concentration and the velocity of air flows into the region intermediate products CO. The CO mass fraction full of surplus gas are two factors which affect the distributions inside the boiler furnace under conditions of combustion rate in the area. The high oxygen concentration different primary to secondary air ratios are shown in Fig. helps to improve the combustion rate. On the contrary, the (8). With the amount of wind increasing, the length of the excessively fast velocity of excessive air hindered the CO mass fraction distribution area becomes shorter in the improvement of the combustion rate. These two factors inner flames but longer in the outer flames. This is because restrain each other and jointly affect the combustion rate. In there is more primary air ejected from the primary air tube this paper, the zone where the combustion reaction is the and less secondary air around the outer flames, it accelerates most drastic is defined as the ideal reaction zone. For the combustion reaction of gas from center gas tube but premixed gas burner, when the opening degree of secondary slows down the reaction of gas ejected from the annular gas air door is relatively small like 15°, the factor of oxygen pipe But generally speaking, the combustion efficiency is the concentration isn’t ideal but the factor of flow velocity is in highest when the proportion of primary air is 15.000%. the ascendant in the central area of line A-B. This causes the Variations of the CO mass fraction of line A-B under faster combustion reaction and an ideal reaction zone more closed to the nozzle of the combustion chamber. While conditions of different primary to secondary air ratios are opening degree of secondary air door increases to a certain shown in Fig. (7b). Although the oxygen concentration in extent, the oxygen concentration couldn’t cooperate with the the central region when the proportion of primary air is velocity of air in the ideal state. So that the combustion rate 7.925% is higher than that when the proportion is 5.985%, in the central area drops down and the ideal reaction zone is the velocity of primary air is faster, so the gas and air here do not have sufficient time to mixed and the combustion moving away from the combustion chamber nozzle. Continuing to increase the opening degree of secondary air reaction is lower than that when the proportion is 5.985%. door, the influence of oxygen concentration is greater than The velocity of primary air in the central area when the that of the airflow velocity, so the combustion rate in this proportion of primary air is 15.000% is faster, however, the region rises up and the ideal reaction zone becomes closer to combustion reaction is much faster than that when the the nozzle of the combustion chamber again. Through proportion is 7.925% because of the higher oxygen concentration. changing the opening degree of secondary air door, users can adjust the position of the ideal reaction zone to control the flame shape. 10 The Open Fuels & Energy Science Journal, 2015, Volume 8 Chen and Liu

3.5. NOx Emissions velocity of airflow is slower, so there is large amount of thermal NOx generated here. And the situation at the end of Firstly, the simulation results of the NO mass fraction x the furnace is formed via the accumulation of thermal NOx and the generation rates of NOx and thermal NOx when the generated in the flowing process of the high temperature flue proportion of primary air is 7.925% are analyzed. gas. Although generation rates in the middle of the furnace is According to the relative theoretical analysis [2-10], fuel high, the velocity of the high temperature flue gas is so quick NOx can be neglected because there is little organic nitrogen that there is no sufficient time to generate NOx and the little in LNG. Prompt NOx, also known as transient nitrogen amount of NOx generated is also taken part from the high oxides, is formed due to the presence of the carbon- temperature flue gas. So the thermal NOx concentration is containing radicals in the flames at low temperature when less here. nitrogen fuel is burning. It is generated mainly in the Distributions of the NOx generation rate and the thermal combustion chamber and this formation mechanism of NOx formation rate at different primary to secondary air prompt NOx is not a major source of nitrogen oxide ratios are shown in Figs. (11, 12), respectively. When the emissions. Therefore, thermal NO is generally considered as x prompt NOx can be neglected, distributions of the NOx the main NOx produced during the liquid natural gas production rate basically agree well with distributions of the combustion process. Thermal NO is formed when nitrogen x thermal NOx formation rate in the furnace interior. It also and oxygen within the combustion gas combine at a confirmed the inference obtained previously that thermal relatively high temperature of 1700 K above for a long time. NOx is the major nitrogen oxides produced in LNG So it is generated inside the boiler furnace. Therefore the combustion process. simulation of nitrogen oxides emission during the LNG The NO concentration contours at different primary to combustion process focuses on the thermal NOx. x secondary air ratios along line A-B are shown in Fig. (10a). At both ends of the area where the furnace and the burner Comparing with the situations in Figs. (4, 7), the result nozzle are connected, the temperature is high and the shows that there are lower temperatures, air speeds and

Fig. (9). Mass fraction distributions of NOx at different primary to secondary air ratios. Numerical Investigation on the Flow, Combustion and NOx Emission The Open Fuels & Energy Science Journal, 2015, Volume 8 11

Fig. (10). Variations of NOx mass fractions along line A-B (a) and line C-D (b).

Fig. (11). Generation rate distributions of NOx at different primary to secondary air ratios. oxygen concentrations in the internal area of flames, so a Furthermore, this NOx generated will be taken away by small quantity of NOx is generated here. airflow, so the amount of NOx generated in this area is the 12 The Open Fuels & Energy Science Journal, 2015, Volume 8 Chen and Liu

Fig. (12). Generation rate distributions of thermal NOx at different primary to secondary air ratios. least at all the three primary to secondary air ratios. When Distributions of the NOx mass fraction at different primary the ratio of primary air is 5.985%, there is more amount of to secondary air ratios are shown in Fig. (9). It can conclude that combustion-supporting secondary air outside the flames but both the total mass fraction and the generation rate of NOx less residence time for the high-temperature gas due to the decrease with the increment of the amount of primary air at both fast velocity, so the minimum mass of NOx is produced. ends of the area where the furnace and the burner nozzle are When the proportion of primary air is 15.000%, the amount connected by comparing with the situations in Fig. (11). This is of secondary air is the least. But the lowest velocity makes because as the amount of primary air increases, the swirling the high-temperature gas can stay for sufficient time. And strength of primary air and overall speed of the airflow the two factors at the proportion at 7.925% are moderate, so increases, so residence time for the high temperature flue gas is there are almost the same total amounts of NOx produced at reduced. Furthermore, oxygen concentration at both ends these two proportions of primary air and just some decreases due to the reduction of the amount of secondary air. differences of concentration distributions depending on the These reasons all reduce the generation of NOx. The distribution distributions of air flow field. situations of the mass fraction and the generation rate of NOx have a close relationship with the flame shape and the The distributions of NO concentration in the center axis x generation rate of thermal NO decreases significantly in the line C-D of the furnace are depicted in Fig. (9b). The mixed x combustion gas near the nozzle of the burner is in the fuel- regions where the temperature is less than 1700 K. rich situation, there is no difference of the amounts of NOx When the proportion of primary air is 15.000%, the flames generated at the three proportions of primary air. In the fuel- are the most concentrated and stable and their heating effect are lean regions, oxygen concentrations have significant the best. Meanwhile, the distribution regions of the NOx influences on the generation of NOx, so the maximum generation rate and the mass of NOx produced are also the amount of the NOx is generated when the proportion of smallest. On the contrary, heating efficiency is the worst and the primary air is 15.000%. amount of NOx generated is the largest when the proportion of primary air is 5.985%. Therefore, the most appropriate primary Numerical Investigation on the Flow, Combustion and NOx Emission The Open Fuels & Energy Science Journal, 2015, Volume 8 13 to secondary air ratio should be chosen through considering ACKNOWLEDGEMENTS comprehensively the situations of heating efficiency and nitrogen oxides emissions which are according to the This work was partially supported by the Innovation combustion process of the premixed gas burner. Foundation Program (No. 2013J4400223) and Business Incubator Project (No. 2013J4200015) from Technology and Information Bureau of Guangzhou City, China. 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The authors confirm that this article content has no conflict of interest.

Received: June 11, 2014 Revised: October 23, 2014 Accepted: October 24, 2014

© Chen and Liu; Licensee Bentham Open.

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